Production of uranium dioxide

ABSTRACT

Uranium dioxide with low fluoride content and excellent milling characteristics suitable for making ceramic grade UO2 pellets is produced by initially hydrolyzing uranium hexafluoride in a fluidized bed of UO2F2 particles at a temperature in the range between 500*F. and 900*F. and preferably between 650*F. and 750*F. to produce initially substantially nonhygroscopic freeflowing uranyl fluoride powder having an average particle size of +60 mesh. The uranyl fluoride thus obtained is subjected to at least two separate pyrohydrolysis treatments in fluidized beds with gaseous mixtures of hydrogen and steam to produce uranium dioxide. In the initial pyrolysis treatment, the ratio of hydrogen and steam is adjusted to provide a mixture with a relatively low defluorination potential and the flow rate of the mixture is also regulated to suppress the rate of defluorination. The resident time of the particle in the fluidized bed for the first pyrolysis treatment is sufficiently long so that about 98 percent of the fluoride in the feed is removed. The incompletely defluorinated particles are then further treated in a second fluidized bed with a hydrogen and steam gaseous mixture having a higher ratio of H2/H2O which will provide a higher defluorination potential and the gas rate is also preferably increased to reduce the partially defluorinated feed to uranium dioxide with a fluoride content less than about 100 ppm and preferably below 40 ppm.

United States Patent [191 Rode [11] 3,765,844 [451 Oct. 16, 1973 1PRODUCTION OF URANIUM DIOXIDE James A. Rode, St. Louis, Mo.

[73] Assignee: United Nuclear Corporation,

Elmsford, N.Y.

22 Filed: Oct. 4, 1968 [21] Appl. No.: 765,071

Related US. Application Data [63] Continuation-impart of Ser. No.741,923, July 2,

1968, abandoned.

[75] lnventor:

[52] US. Cl 423/19, 423/253, 423/258, 423/261 [51] Int. Cl. C0lg 43/02[58] Field of Search 23/326, 346, 352, 23/355; 252/30l.l

Primary ExaminerCarl D. Quarforth Assistant ExaminerF. M GittesAttorney-Pennie, Edmonds, Morton, Taylor and Adams [5 7] ABSTRACTUranium dioxide with low fluoride content and excellent millingcharacteristics suitable for making ceramic grade U0 pellets is producedby initially hydrolyzing uranium hexafluoride in a fluidized bed of UO Fparticles at a temperature in the range between 500F. and 900F. andpreferably between 650F. and 750F. to produce initially substantiallynonhygroscopic free-flowing uranyl fluoride powder having an averageparticle size of +60 mesh. The uranyl fluoride thus obtained issubjected to at least two separate pyrohydrolysis treatments influidized beds with gaseous mixtures of hydrogen and steam to produceuranium dioxide. In the initial pyrolysis treatment, the ratio ofhydrogen and steam is adjusted to provide a mixture with a relativelylow defluorination potential and the flow rate of the mixture is alsoregulated to suppress the rate of defluorination. The resident time ofthe particle in the fluidized bed for the first pyrolysis treatment issufficiently long so that about 98 percent of the fluoride in the feedis removed. The incompletely defluorinated particles are then furthertreated in a second fluidized bed with a hydrogen and steam gaseousmixture having a higher ratio of Hg/HZO which will provide a higherdefluorination potential and the gas rate is also preferably increasedto reduce the partially defluorinated feed to uranium dioxide with afluoride content less than about 100 ppm and preferably below 40 ppm.

10 Claims, 1 Drawing Figure PAIENIEDum 16 I973 SHEET 10F 2 FIG. I

R E B B U m R A F H m 0 R T O T C A E R WEIGHT RECORDING INDICATOR N2STEAM UF VAPORIZER INV'ENTOR JAMES A. RODE BY ?Qum; dmuwls mmm ,Ta afllATTORNEYS PRODUCTION OF URANIUM DIOXIDE CROSS-REFERENCE TO RELATEDAPPLICATION This is a continuation-in-part application of my copendingapplication Ser. No. 741,923, filed July 2, 1968, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to uranium dioxide and, more particularly, to a new process forproducing the same.

2. Description of the Prior Art The Argonne National Laboratorydeveloped a dry process for producing U0, from UF utilizing fluidizedbeds to convert uranium hexafluoride to uranyl fluoride by hydrolysisand then to transform the latter by pyrohydrolysis to uranium dioxide(AEC Reports AN- L-6023 and ANL-6902). In the hydrolysis operation theuranium hexafluoride-steam reaction was carried out continuously at arelatively low temperature, about 400 to 450F., and uranyl fluoride thusobtained is extremely hygroscopic which tends to deposit on internalsurfaces of the reactor. The deposition of uranyl fluoride on thereactor vessel requires periodic shutdown for the removal of thedeposits, thus rendering the process not suitable for commercialapplication.

To correct this problem, a high temperature process in which thehydrolysis reaction is carried out at 932F. was proposed. At the hightemperature, the UP conversion product is not granular and contains UAlthough the product formed at the high temperature process exhibits nodeliquescence in contrast to the deliquescence of low-temperaturematerial, the proposed high temperature process failed in continuousoperation due to excess fines generation and poor particle control inthe fluidized bed.

According to the Argonne process, the reduction of the uranyl fluorideto the oxide was carried out in a fluidized bed at a temperature in therange between ll00F. and l200F. with a 50-50 mixture of steam andhydrogen for a period of four to seven hours. The product produced inthe Argonne process, however, is not suitable for the fabrication ofceramic U0 pellets due to high fluoride content (above 150 ppm evenafter four to seven hours of treatment) and high nickel contaminationdue to the corrosion of the reaction vessel at a severe temperature andcorrosive atmosphere.

SUMMARY OF THE INVENTION I have discovered that uranium dioxide withextremely low fluoride content, i.e., to 40 ppm (parts per million) andnickel contamination can be produced in a continuous multi-stageoperation. Broadly stated, the process of this invention comprisesintroducing separately into a reaction zone having therein fluidizeduranyl fluoride particles, steam and gaseous uranium hexafluoride, thelatter at a rate and in a condition sufficient to prevent the formationof uranium fluoride solids prior to its reaction with the steam in thefluidized reaction zone. The concentration of the gaseous uraniumhexafluoride and steam is regulated to promote a surface reaction on thefluidized particles while maintaining the temperature in the aforesaidrange. Additional uranyl fluoride seed advantageously is added to thefluidized reaction zone to stabilize the average particle size thereinand the excess uranyl fluoride is periodically or continuously removedtherefrom.

Preferably an inert gas such as nitrogen and carbon dioxide is used as adiluent for the reaction. The diluent may be advantageously used tofluidize the bed either alone or in combination with the steam used forthe hydrolysis. The gaseous diluent is also found to be suitable as acarrier for gaseous uranium hexafluoride.

The uranyl fluoride thus obtained is reacted in a second fluidized bedwith a gaseous mixture of hydrogen and steam at a temperature in therange between ll00F. and 1450F. for a period sufficient to reduce amajor portion of the fluoride. The defluorination conducted in thesecond fluidized bed is controlled by regulating the flow rate of thehydrogen-steam mixture fed into the fluidized bed and the ratio of Pi /HO therein to provide a mixture with a low defluorination potential and ahydrogen concentration therein to below about 50 percent in excess ofthe stoichiometric requirement. The ratio of H /H O of the gaseousmixture preferably is below about 30 percent. The product from thesecond fluidized bed is subjected to at least one additionaldefluorination treatment with a gas-mixture having a Pi /H O ratiohigher than 30 percent and using a gaseous flow rate sufficient toproduce uranium dioxide having a fluoride content less than about 100ppm and preferably below 50 ppm. The defluorination in the additionaltreatment is in the range of l 100F. to l450F.

The present invention is based on the discovery that continuoushydrolysis of uranium hexafluoride in a fluidized bed to form highdensity and non-hygroscopic uranyl fluoride can be carried out if (a)the temperature is carefully controlled to be within the range of 500F.to 900F. and preferably between 650F. and 750F., and (b) theintroduction of the gaseous uranium hexafluoride is maintained at asufficiently high velocity so that the transformation of UF to othersolid intermediate fluorides of uranium such as UF does not occur priorto its reaction with the steam in the fluidized bed or the prematurereaction of steam with UF within the nozzle. The latter discovery isparticularly significant because at temperatures above 500F.,transformation of UF to crystalline uranium fluorides occurs at a ratesufficiently high as to causeplugging of the UF inlet nozzle, unless theUP inlet velocity is controlled.

The uranyl fluoride produced in the initial step of the process of thisinvention is significantly less hygroscopic. It has substantiallyuniform particle size of about +60 mesh, is free-flowing, has high bulkdensity and can be handled in atmosphere. The high density UO F thusproduced is particularly suitable for subsequent pyrolysis treatments toform U0 for reasons which will be apparent from the followingdescription.

In addition, a major advantage in the initial step of the presentinvention over the prior art is the drastic reduction in the amount offines recycle required to maintain the particle size stability. Therecycle is less than about 10 percent by weight of the total fluidizedbed and is generally in the range of 0 to 2 percent in order tostabilize a bed having percent to 98 percent by weight of +60 meshparticles. As a comparison, in processes conducted at 446F. the finesrecycle was in the range of 14 to 24 percent by weight. A large finerecycle is undesirable partly because it represents nonproductive use ofprocess capacity, but more importantly, the need forsuch a large amountof recycle indicates severe inherent process instability. Large recycletends to perturb the steady-state conditions in the reaction, lowers thetemperature in proportion to the weight of the recycle and threatenscomplete loss of control of process step.

The invention is also based on the discovery that the fluoride contentin the uranium dioxide can be substantially reduced when a multi-stage,instead of a single stage, defluorination operation is used. Contrary toexpectation, the total resident time of the uranium particles in thereaction zones is less and the combined hydrogen utilization efficiencyis substantially higher in the multi-stage operation of this inventionthan in the prior one-step defluorination process. Another significantadvantage of the present multi-stage defluorination operation is in itscapability to produce, from the high density hydrolysis product, uraniumdioxide particles with high internal stress and strain which can besubsequently milled with less energy than particles produced in theone-step operation. Not less important for commercial adaptation of thisprocess is the fact that the multi-stage operation of this inventionsubstantially eliminates the corrosion problems which contribute to highnickel contamination in the prior art process, even at operationtemperatures (i.e., up to l450F.) higher than those heretofore used.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a flow sheet for thehydrolysis of uranium hexafluoride to uranyl fluoride and;

FIG. 2 shows a flow sheet for the multi-stage defluorination operation.FIGS. 1 and 2 combine to form a single flow sheet to illustrate thepreferred process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of thisinvention, the chemical reaction of the initial step, the hydrolysis ofuranium hexafluoride, may be represented by the following equation:

AH 8F= 26.8 kcal/mole The exothermic reaction can be carried out over awide temperature range. At temperatures below 500F. the hydrolysisproduct is extremely hygroscopic. As the temperature of the reactionincreases above 500F., the deliquescent characteristic of the resultanturanyl fluoride diminishes. I found the tendency of UO F- to deposit onthe walls of the reaction vessel is reduced to an acceptable level whenthe reaction is above 500F. However, for the process of this invention Iprefer to carry out the hydrolysis at a temperature in the range between650F. and 750F. Within this temperature range the deposition problem issubstantially eliminated. Furthermore, at this higher temperature range,the heat generated from the exothermic reaction is more usable andconvenient to remove than the heat from the low temperature process.

For a continuous hydrolysis operation in a fluidized bed, it isessential that the concentrations of the reactants in the fluidized bedare such that, at the preferred temperature range, the reaction ispredominantly a coating type or surface induced reaction. With this typeof reaction, the uranyl fluoride from the hydrolysis is produced in theform of a coating on the UO F seed in the fluidized bed. Attrition ofthe large particles of tines generated by a minor vapor-phase operationproduces additional seed or surface area for the reaction to replace thesurface area that is lost by particle growth and by the particlesremoved from the bed. At the optimal operation condition, the seedgeneration and particle growth should be balanced to provide astabilized fluidized bed operation with little or no addition of seedsinto the fluidized bed.

I have found that a stable operation can be maintained in the preferredtemperature range when the amount of steam used is above thestoichiometric requirement for the hydrolysis but is not in excess of300 percent. The amount of steam to be used in the reaction depends alsoon the amount of gaseous diluent in the reaction zone. Generally, asatisfactory operation condition can be obtained with less than 200percent of excess steam when the amount of gaseous diluent is in therange of 0 to 50 percent of total gas volume in the fluidized bed.

The use of inert gaseous diluent such as nitrogen or carbon dioxide isessential for the successful operation of this invention when thecapacity of the equipment inherently limits a high feed rate of LIF intothe fluidized bed. At the reaction temperature of above 500F., thegaseous UF has a tendency to transform into crystalline lower fluoridessuch as UF before it can react with steam, thus plugging the inlet UFnozzle. To avoid plugging, a high gas flow rate must be used. I found aminimum flow rate of 5000 cfh per sq. in. of nozzle opening (36 cfh fora 3/32 in. diameter nozzle) is required in order to have a plugging freeoperation. Since the coating reaction is dependent on the concentrationsof the reactants and the minimum resident time of UF in the reactor, ahigh flow rate oftentime cannot be used with an average capacityfluidized bed reactor. By using an inert diluent, however, the amount ofUF introduced into the reactor can be reduced without limiting thenozzle velocity of the feed.

As shown in FIG. 1, the UF gas from the vaporizer which may be premixedwith N is fed into the lower part of the fluidized bed of uranylfluoride particles after passing through the weight recording indicator.The temperature of UP or its admixture with nitrogen should be above thecrystallization temperature of UF at the pressure used to inject thefeed into the fluidized bed. Generally, a temperature in the range of200F. to 300F. is suitable. The pressure required to inject the gaseousmixture into the fluidized bed at the minimum nozzle velocity depends onthe nozzle design and other variables. I have found a pressure in therange of 5 to 15 psi to be sufficient for achieving the minimum nozzlevelocity.

The steam may be separately introduced into the fluidized bed reactor.Advantageously, steam alone or in combination with N is used to fluidizethe bed of UO F particles. The steam or the mixture of steam and N ispreheated to a temperature in the range of 500F. to 700F. and then isfed into the bottom of the column to fluidize the bed of UO F particles.Depending on the height of the fluidized bed, a pressure in the range of5 to 10 psi is normally sufficient for this purpose.

In starting the hydrolysis operation, a bed of UO F particles having aparticle size in the range of 60 to mesh is initially heated to theoperational temperature by external heating coils. After the operationaltemperature has been achieved, steam is initially fed into the reactorfor a period of about 10 minutes to an hour and thereafter the UF gasmixture is introduced into the reactor to react with the steam. Sincethe hydrolysis is exothermic, the reactor temperature may be maintainedby using external cooling means. For a large reactor, the loss of heatfrom its surface may be sufficient to avoid external cooling coils.

At the beginning of this operation, the UO F particles are withdrawnperiodically to maintain a proper bed height. A portion of these UO Fparticles is recycled initially for the purpose of stabilizing theparticle size to within the range of 30 to 60 mesh. After the averageparticle size of UO F, in the fluidized bed reaches the desired particlesize, the product is removed periodically or continuously for furtherprocessing to produce U0, by pyrohydrolysis according to the procedureto be described hereinbelow.

To avoid excess growth of the UO F particles to above 30 mesh, a certainamount of uranyl fluoride particles may be advantageously milled to aparticle size in the range of 60 to +l00 mesh to serve as seed in thefluidized bed when the generation of fine is not sufficient for thestabilization of particle size. Generally I found a recycle rate afterstabilization is attained of less than percent by weight of the bed andcan be controlled to below about 2 percent.

The fluidized bed reactor is equipped at the uppermost portion withfilters which are used to entrap the fines carried up by the off-gas. Aperiodic blowback of the filter is used for cleaning and the finesfalling back to the bed serve as additional seed. The off-gas containingsteam, nitrogen and HF is sent to an HF scrubber wherein the HF gas isrecovered as hydrofluoric acid.

According to the present invention, uranyl fluoride produced byhydrolysis of UF is defluorinated by pyrohydrolysis with hydrogen andsteam in a multistage operation. The exact mechanism of the reduction isnot yet certain. The defluorination may follow the first or both modesof reactions represented by the following equations:

In the prior Argonne study (referred to hereinabove), the fluorideremoval was found to be incomplete and slow when hydrogen alone was usedto reduce uranyl fluoride, due possibly to the formation of UF,according to the following reversable reaction:

In the same study it was found that a 50-50 mixture of hydrogen andsteam provides the optimal rate for defluorination.

In the multi-stage defluorination of UO,F, according to the presentinvention, a fast defluorination rate in the first defluorination stepis detrimental to the success of the present process because of thegeneration of excess fines which, as stated hereinabove, would preventthe continuous operation of the fluidized bed reactor. As stated above,the high density uranyl fluoride particles are produced in thehydrolysis operation essentially by a coating type of process; hence,these particles have an onion-like internal structure. Inpyrohydrolysis, UO,F, particles when first coming into contact withhydrogen and steam react with them rapidly. The reaction rate, however,decreases after the first layer is converted to the oxide form at whichtime the reaction rate is dependent to a large extent on the diffusionrate of the gaseous reactants in the particles. In the transformation ofUO F, to the final oxide form internal stress and strain are impartedinto the particles due to phase transformation UO F to U0 and densitydifferential between these two compounds. The fine generation is causedby excess buildup of internal stress and strain resulting from a highdefluorination rate at the beginning of the hydrolysis.

Defluorination of UO F at a high rate is also not desirable because itcauses an excess retention of HF in the reaction zone. A highconcentrationof HF favors the formation of UF, and hinders the completeconversion of UogFg according to the equation:

UO 4HF 211 0 UF4 A high concentration of HF at the reaction temperaturealso causes higher corrosion rate of the reactor vessel, thusintroducing a larger amount of contamination.

The internal stress and strain, however, may be advantageously utilizedif they can be retained within the U0 particles without causingattrition or breaking up of these particles. I found the internal stressand strain may be retained within the particles when the defluorinationrate is controlled. The retained stress and strain in the U0 particlescan be subsequently milled with less energy. A saving of milling energyup to about 40 percent to 50 percent may be attained in fluid mill withU0 particles produced according to the process of this invention ascompared to similar U0 particles prepared without the retained stressand strain.

The controlled defluorination in the first step of pyrohydrolysis isachieved by using a hydrogen and steam mixture with relatively lowdefluorination poten tial which comprises less than about 30 percent ofH by volume and preferably in the range between 20 per-- cent to 25percent to defluorinate uranyl fluoride in a fluidized bed. The gaseousmixture should be fed into the bed at a rate sufficient to provide' anequivalent amount of resident H in the fluidized bed slightly in excessof the stoichiometric requirement (based on equation (I) presentedhereinabove). Preferably the excess H in the' bed is less than about 50percent. Using this gaseous composition and with the rate stated above,I found a major portion of fluoride (in excess of can be reduced in arelatively short period of time at a temperature in the range of llO0F.to 1450F. The partially defluorinated particles thus produced haveretained stress and strain and when properly reduced in subsequentdefluorination operations, possess excellent milling characteristics.

Further to describe the multi-stage defluorination of this inventionreference is now made to FIG. 2.

As shown therein, UO F from the hydrolysis reactor (shown in FIG. 1) isfed either continuously or intermittently into the first of the twodefluorination fluid bed reactors which are constructed in the samemanner as the hydrolysis reactor with each having a bottom gasdistribution section, a cylindrical main fluidizing section and an uppergas disengaging section. Due to severe operation conditions, thesereactors are fabricated, preferably with materials having highresistance to corrosive hydrofluoric acid at extremely elevatedtemperatures. Alloys such as lnconel may be advantageously used. Thefluidized reactors are equipped with external heaters (not shown) forheating or maintaining the reaction at the proper temperature range ofll00F. to 1450F.

The uranyl fluoride particles are fed into the fluidized bed reactorthrough a feed port located at the fluidizing section of the reactorwherein a bed of U0, particles is fluidized by steam and hydrogenentering from the bottom gas distribution section. Both the steam andhydrogen may be from standard commercial sources. Preferably these gasesare preheated to a temperature above 500F. prior to their introductioninto the bottom portion of the reactor. Flow meters are used to controltheir respective flow rates and the ratio of H and H used for thedefluorination.

Advantageously, a source of N may be connected to the bottom section ofthe reactor. The nitrogen is used to start up the reactor and as adiluent for the steam and hydrogen mixture to control the reaction rateduring the operation of the reactor. Usually I found that the use ofadiluent is not needed once the fluidized bed is stabilized.

As stated hereinabove, the success of the present process is dependentto a large extent on contolling the defluorination rate of the uranylfluoride. At the defluorination temperature in the range between ll00F.and 1450F., the hydrogen-steam gaseous mixture stated above provides amoderate defluorination rate when the H in the fluidized zone is notsubstantially above the stoichiometric requirement. Usually, the amountof excess hydrogen should not be more than 30 percent and preferably notabove 20 percent. By deliberately limiting the defluorination potentialof the hydrogen and steam, the reduction of UO F is sufficiently slow sothat the fragmentation of the particles is limited to a tolerableextent. The unfragmented particles retain the stress and strain whichafter subsequent defluorination treatment can be milled with lessenergy.

Even by limiting the reduction rate I found the defluorination proceedsrapidly. The uranium particles generally have a resident time less thanabout three hours and generally about one hour before the fluoridecontent therein is lower than about 2 percent. In this first stage ofdefluorination, the temperature is preferably maintained at the upperlimit of the temperature range, i.e., l300F. to 1400F.

The uranium dioxide from the first defluorination reactor (I) iswithdrawn therefrom either continuously or intermittently through aproduct exit port which may be located in the bottom section or at themid section of the reactor. The latter arrangement collects the overflowas the product. The percentage of the fluoride converted to the oxideform in the product is dependent on the resident time of the feed andthe reaction conditions. The product may contain a higher percentage offluoride when more than two stages are used for the defluorination. Insuch instance, the intermediate stage or stages may be used to reducethe fluoride with a gas mixture with gradually increasing defluorinationpotential. When a two-stage process is used, however, the amount offluoride retained in the first stage product should be sufficient sothat a gas mixture with a high deflyorination potential may be used inthe latter stage without causing fragmentation or excess l-IFconcentration. From my experiments, a fluoride content of 3000 ppm inthe first stage product is preferred.

The first stage product is further defluorinated in a second fluidizedbed reactor (II) in substantially the same manner as in the firstdefluorination reactor. Since a major portion of the fluoride has beenreduced, the concentration of H in the hydrogen and steam mixture in thesecond defluorination reactor is enriched and the flow rate of thereaction gases is increased to insure a substantially completedefluorination. I found the H content in the reaction gaseous mixturewhich provides optimal results is in the range between 30 to 50 percent.The flow rate of these gases, however, should be such that the excess His below about 100 percent of the stoichiometric requirement. Thedefluorination in the second stage should be in about the sametemperature range as for the first stage. Defluorination of U0 to below50 ppm of fluoride at these reaction conditions generally requires lessthan about three hours and usually about one hour of U0 resident time inthe reactor.

As shown in FIG. 2, the off-gases from both reactors which contain HF,unreacted H and steam, as well as entrained fines, are treated torecover the HF as hydrofluoric acid and the fines for recycling as seedsfor the hydrolysis reactor.

Instead of UO F as the starting material for the defluorination, UF, maybe used. When uranium tetrafluoride is used, the defluorination proceedssubstantially in the same manner as described. The defluorinationtemperature should be slightly lower in the initial defluorination stageto avoid possible sintering of UF,.

The U0: produced according to the process of this invention can befluid-milled to active oxide for the production of ceramic grade U0pellets.

Further to illustrate this invention, a specific example is describedhereinbelow. In this example all three fluidized bed reactors weresimilarly constructed. They are fabricated from Inconel cylinders havingan interior diameter of about four inches and equipped with two filtersat the uppermost portion. The fluidizing section of these reactors isabout 6% feet long. In the hydrolysis operation, the UF feeds were mixedwith 25 to 50 percent by volume of N The temperature of the feeds was230F. The steam used for the reaction and the fluidization was alsopremixed with about 25 percent by volume of N and the mixture waspreheated to 600F. before feeding into the reactor.

EXAMPLE In the first run the reactor was loaded with a 35 pound chargeof percent+60 me sh UO2l 2, 17 percent and 3 percent +200 mesh whfiw asmaintained at 700F. The UF flow rate was stabilized at l l to 13 poundsper hour. During the first five hours of the run, the +200 fractionincreased to 41 percent of total bed weight. The high increase in the+200 was in all probability caused by a recycle of 1.0 pound per hour.When the recycle rate was reduced to 1.0 pound per every two hours toreduce the growth of +200, the +60 fraction increased to 89 percent fivehours after changing recycle rate. At this time the bed contained 10.6percent +100 mesh and 0.4 percent +200 mesh UO F The +60 fraction wasallowed to increase to 96 percent before particle growth was stabilized.The +60 fraction was maintained at 96 percent for approximately sixhours.

The second was an extended 55 hour run which was made utilizing theinformation derived in the previous run. The reactor was operatedcontinuously with normal filter, reactor and diffuser plate pressurethroughout the run. A UF flow rate of l 1 pounds per hour wasmaintained. After approximately eight hours of operation the particlesize had stabilized with the product analyzing 97.5 percent +60 and 2.5percent +100. A recycle rate of 0.5 to 1.0 pound of 60 UO F recycledevery two hours was used to hold the +60 fraction to 95 to 97 percent.Because of the rapid growth of +30 mesh particles from +60, fluidizationbecomes difficult when the +60 fraction exceeds 97 percent of total bedweight, consequently, 60 mesh UO F was recycled specifically to slowdown the growth of +30 mesh particles. After 55 hours of operation, theproduct analyzed 96 percent +60, 3.5 percent +100 and 0.3 percent +200.

1n the third run the reactor was charged with 35 pounds of UO F 80percent +60, 16 percent +100 and 4 percent +200. Observations duringprevious runs had shown that fluidization was easily afiected when thebed contained some fine particles at start-up.

The object of the run was to demonstrate particle size control overextended periods of operation utilizing the information learned in theprevious run with variations in the UF flow up to 25 pounds per hour.The UF flow was started and stabilized at 19 pounds per hour forapproximately two hours; it was then increased to 25 pounds per hour.

During the first two to three hours of the run, the +60 fractionincreased to 96.5 percent with a corresponding decrease of fines to 3.2percent +100 and 0.3 percent +200. The recycle rate was increased to0.75 pound every 2 hours when screen analyses showed 15 percent +30 meshUO F in the bed, which was used to limit the +30 particles.

At the end of the run, screen analyses showed 97.8 percent +60, 2.2percent +100 and 0.3 percent +200.

The uranyl fluoride produced in these runs are used to prepare U0,. Thefirst reactor was preheated to 1300F. before the 45 pounds of UOJ, wasfed thereof which was 60 +100 mesh. After three hours of pyrohydrolysis,the average fluoride level was reduced to 1000 to 5000 ppm and cyclicaloperation was initiated. Regular withdrawals of 17.5 pounds of U0, werefollowed by charging in 20 pound increments to maintain a bed of 40pounds of U0,.

The hydrogen flow of only 20 percent in excess of stoichiometricrequirements was deliberately used to limit the rate of fluoride removaland thereby limit the HF concentration in the stack gas which couldcause conversion of U0, to UP. and minimize the fluoride concentrationgradients within the particles (large fluoride gradients result indensity gradients which in turn produces a fragile particle andexcessive attrition during fluidization). The process removed 98 percentof the fluoride. Process parameters are defined in the attached tableunder runs 1 and 2.

The second step pyrohydrolysis of the product from runs 1 and 2 wasinitiated with drastically increased gas flows to assure adequatefluoride removal (Run 2.1) using a gas mixture containing 40% P1,.Excessive attrition and solids entrainment in the gas stream resulted.Since only trace levels of fluoride were present in the feed, the steamflow was reduced with no hazard of hydrofluorination of the U0,. Inspite of the higher hydrogen concentration, the total hydrogenconsumption was held to 100 percent excess for run 2.2 with a fluoridelevel of 10 ppm.

The product was micronized and blended producing a finely divided U0,analyzing 42 percent l.4p. by sedimentation and 0.65 p. by Fishersub-sieve size analysis. The oxide was agglomerated with PVA, pressedand i t th h th f d port, Nitr wa i tr d d sintered to produce sound,specification density pellets.

TABLE I Charge Withdrawal Temperature, Frequency Frequency degrees F.Gas flow. s.c.h.f. Fluoride Feed Bed wt., Pass average, Produce particleRun No product lbs. Cycle/hr. Lbs. Cycle/hr. Lbs. Max. Min. N2 Hz H1ONo. p.p.m. size Run 2.1-U02 broke up during second step ofpyrohydrolysis and was blown over to the cyclone as a result of veryhigh gas flows. Run 2.2-Represents ideal conditions for second step ofpyrohydrolysis.

Summary of run data.

into the bed to maintain fluidization until the reaction temperature of1300F. was reached. At this point, steam and hydrogen flows were startedand the nitrogen was shutoff. Steam and hydrogen at a ratio of threeparts steam to one part hydrogen were used, both to reduce the uranylfluoride and to fluidize the bed. in the first reactor the U0 1: wasreduced to U0 with a fluoride content in the range of 3500 to 2500 partsper million with an average bed resident time of three hours per poundsof uranyl fluoride.

The original 45 pound bed of uranyl fluoride was reduced for three hoursprior to the beginning of additions of 20 pound batches of U0 1} orremoval of U0 I claim:

1. A process for the conversion or uranium hexafluoride by hydrolysisfirst to uranyl fluoride and then to uranium dioxide which comprises:

introducing steam and gaseous uranium hexafluoride formation of uraniumfluoride solids prior to its reaction with the steam in the fluidizedreaction zone:

regulating the relative concentration of the gaseous uraniumhexafluoride and steam in said first reaction zone so that the amount ofsteam is greater than but does not exceed three times the amounttheoretically required for the hydrolysis reaction, while maintainingthe temperature therein above 500F. but below 900F. to promote a surfacereaction on the fluidized uranyl fluoride particles;

maintaining sufficient uranyl fluoride seed particles in the reactionzone to stabilize the average particle size therein at a level such thatat least about 80 percent of the particles are larger than +60 mesh(Tyler standard) removing excess uranyl fluoride particles from thereaction zone;

reacting the excess uranyl fluoride particles in a second reaction zonewith a hydrogen and steam mixture at a temperature between 1 100F. and1450F. to remove at least 80 percent of the fluoride in the uranylfluoride, the amount of hydrogen used being lower than 50 percent abovethe stoichiometric requirement and the ratio of hydrogen and steam inthe gaseous mixture being in the range between 0.20 to 0.30;

subjection the defluorinated uranyl fluoride to at least one moredefluorination treatment with a gaseous mixture of hydrogen and steamhaving a ratio of Hg/HgO higher than the mixture used in the secondreaction zone but below about 0.60 in an amount sufficient to providehydrogen more than 50 percent above the stoichiometric requirement, at atemperature in the range between 1 100F. and 1450F. and for a periodsufficient to produce uranium dioxide with a fluoride content belowabout 100 ppm and recovering the uranium dioxide therefrom.

2. A process according to claim 1 wherein the hydrolysis of uraniumhexafluoride in the first reaction zone is carried out in the presenceof an inert gaseous diluent.

3. A process according to claim 2 wherein the diluent is nitrogen.

4. A process according to claim 1 wherein the temperature in the firstreaction zone is maintained in the range between 650F. and 750F.

5. A process according to claim 1 wherein the gaseous uraniumhexafluoride is premixed with not more than about 50 percent by volumeof N 6. A process according to claim 1 wherein the particle size ofuranyl fluoride in the first reaction zone is stabilized to within therange of percent to 98' percentby w eigh t of mesh by recyclingless than15% by weight of the total fluidized bed of UO,F seed having particlesize in the range between 60 and +100 mesh.

7. A process according to claim 1 wherein the defluorination of uranylfluoride in the second reaction zone is carried out in a fluidized bed.

8. A process according to claim 1 wherein the defluorination of theuranyl fluoride is carried out successively in at least two separatefluidized beds.

9. A process according to claim 1 wherein the defluorination in thesecond reaction zone uses a 0.30 hydrogen and steam mixture and thesubsequent defluorination treatment is carried out using a 0.40 hydrogenand steam mixture.

10. A process according to claim 1 wherein at least a portion of theexcess uranyl fluoride is recovered as a secondary product of theprocess.

2. A process according to claim 1 wherein the hydrolysis of uranium hexafluoride in the first reaction zone is carried out in the presence of an inert gaseous diluent.
 3. A process according to claim 2 wherein the diluent is nitrogen.
 4. A process according to claim 1 wherein the temperature in the first reaction zone is maintained in the range between 650*F. and 750*F.
 5. A process according to claim 1 wherein the gaseous uranium hexafluoride is premixed with not more than about 50 percent by volume of N2.
 6. A process according to claim 1 wherein the particle size of uranyl fluoride in the first reaction zone is stabilized to within the range of 80 percent to 98 percent by weight of +60 mesh by recycling less than 15% by weight of the total fluidized bed of UO2F2 seed having particle size in the range between -60 and +100 mesh.
 7. A process according to claim 1 wherein the defluorination of uranyl fluoride in the second reaction zone is carried out in a fluidized bed.
 8. A process according to claim 1 wherein the defluorination of the uranyl fluoride is carried out successively in at least two separate fluidized beds.
 9. A process according to claim 1 wherein the defluorination in the second reaction zone uses a 0.30 hydrogen and steam mixture and the subsequent defluorination treatment is carried out using a 0.40 hydrogen and steam mixture.
 10. A process according to claim 1 wherein at least a portion of the excess uranyl fluoride is recovered as a secondary product of the process. 